In any industry that utilizes instrumentation to monitor and/or control a process, such as temperature of a fluid or pressure in a tank, it is highly desirable, if not mandatory, to have an indicator of the process variable. Prior art devices include using moving pointer meters and strip chart recorders to monitor the variations in the process signal.
The present state of the art calls for digital panel meters to monitor and display the process variable signal. This type of system has the advantage of being able to display the process variable signal directly in the engineering units desired; for example, pressure or temperature. This is simply a matter of calibrating the meter in units of measured process signals.
Where controls of one type or another are needed, digital panel meters of this type provide alarms which can be set to either a high or low condition to thus provide additional control capabilities which do not require human interfacing in order to be effective. For example, on the scale of zero to one thousand, a process temperature may operate satisfactorily in the range of 300 as a low point and 650 as a high point. An alarm incorporated in the system will be actuated when the process signal reaches a low point of 300. Furthermore, a second alarm may be actuated when the process signal exceeds a high reading of 650 or above. A shortcoming of the prior art systems occurs when the process signal fluctuates about either a high or low preset point. The fluctuations may occur due to various incremental changes in the process variable being monitored or because of noise conditions inherent in the system. Should the process signal fluctuate about a preset high or low point, a series of alarms and alarm resets could occur because of the variations of the process signal from the nominal steady state value.
Additionally, it is often desirable to have considerable control over where the alarm should reset in terms of the process variable which would be different from the original preset alarm figure. This would enable a system to be brought back into its normal operating condition without the equipment cycling on and off around the preset point.
In monitoring control systems of this type, the definition of hysteresis as it relates to this area of control is the difference between the control actuation set point and its reset point. This difference is often expressed as a percentage of the full indicated process range.
It is known in the prior art to prevent fluctuation of a least significant digit in a meter as shown in the U.S. Pat. Nos. 3,551,809 to Dufour; Miller 3,621,391; and Gray 3,728,524. However, this capability has not been heretofore used in a process variable system wherein significant shifting of the reset point is provided to precisely control the set point change.
Most systems of this type use digital alarm comparators which have no hysteresis and, thus, will cause an alarm reset to occur as soon as the least significant digit of the variable data changes out of alarm coincidence. When dealing with physical devices, such as pumps, heaters, volumes of liquid, temperature, and so forth, which by their nature demand latitude in fluctuations in their control, these digital alarm comparators find their usefulness extremely limited. For example, in a digital process metering loop providing either high or low alarm detection of some process variable which is being monitored by an analog based transmitter and which is subsequently converted to a digital signal for both alarm and display purposes, the required hysteresis effect necessitates the use of two alarm comparators of different configurations with external logic circuitry such that one can start the process variable device and the other can stop it. In addition to this, electronic noise present in the signal loop can cause a variation of several digits around the least significant point which is beyond the range of present analog to digital antiflutter circuits thereby causing alarm sets and alarm resets such that the digital alarm comparators used in these situations are useless.